The present invention relates to an improved power source switching device, and more particularly relates to improvements in selective switching of a connection to two different power sources such as an ordinary power source, and an emergency power source, for testing and maintenance of device elements without interception of power supply.
A typical conventional fixed-type switching device is provided with two input terminals and one output terminal. The first input terminal is connected to the first power source, such as an ordinary power source, the second input terminal is connected to the second power source, such as an emergency power source and the output terminal is connected to a load circuit. The switching device is further provided with two contacts. The first contact is used for separable connection between the first input terminal and the output terminal and the second contact is used for separable connection between the second input terminal and the output terminal.
With this construction, the output terminal is manually or magnetically connected to either of the first and second contact for selection of the load circuit to one of the power sources. The first and second contacts are generally interlocked electrically and mechanically in order to avoid concurrent closure.
For testing and/or maintenance of device elements, the switching device fixed to a wall or a post has to be fully disconnected from the power sources. In this case, power supply to the load circuit is inevitably intercepted. When the load circuit includes one or more important electric appliances which allow no interception of power supply, no testing and/or maintenance of the switching device can be carried out.
It is possible to directly connect these electric appliances to the ordinary power source by means of a by-path unit. In this case, however, the two contacts of the switching device must be reliably interlocked with the by-pass unit. Otherwise the two power sources may be incorrectly connected to the electric appliances and/or contacts of the by-pass unit may be seriously damaged by arc discharge.